1. Field of the Invention
The present invention relates to semiconductor crystals for generating terahertz waves, terahertz wave generators incorporating the semiconductor crystals, semiconductor crystals for detecting terahertz waves, and terahertz wave detectors incorporating the semiconductor crystals.
2. Description of the Related Art
Terahertz waves are electromagnetic waves having a frequency in the range of 0.1 to 10 THz (wavelength: 30 to 3,000 μm) and their wavelength substantially overlaps infrared and far-infrared regions. The terahertz region ranging from 0.1 to 10 THz has not been extensively explored in the past. Recently, studies are being conducted to apply the electromagnetic waves in this region to environmental instrumentation such as imaging or tomography, biology, and medical science, and the terahertz waves are thus increasingly gaining importance. In order to expand the applications of the terahertz waves, efficient generation and detection of terahertz waves are indispensable. The present invention relates to semiconductor crystals for generating terahertz waves, terahertz wave generators incorporating the semiconductor crystals, methods for generating terahertz waves using the semiconductor crystals, terahertz wave detectors incorporating the semiconductor crystals, and methods for detecting terahertz waves using the semiconductor crystals.
As the methods for generating terahertz waves, a difference frequency generation technique using a parametric element, a parametric oscillator generation technique using a nonlinear element, a generation technique using a crystal having electro-optic effects (hereinafter referred to as “EO effects”), and the like have been available. Among these techniques, the technique of generating terahertz waves by utilizing the EO effects advantageously has high practicality since the technique allows easy optical alignment.
As the devices for detecting terahertz waves, bolometers, optically conductive antennas, those that utilize electro-optical (EO) effects, such as ZnTe, etc., have been available. Among these, optically conductive antennas and devices that utilize the EO effects are widely used since they do not require cooling of elements and yet achieve relatively high detection efficiency.
As the method for generating and detecting terahertz waves by using ZnTe crystals that exhibit EO effects, a technique developed by J. Shan et al. (J. Shan, A. Nahata, and T. Heinz, “Terahertz time-domain spectroscopy based on nonlinear optics”, J. Nonlinear Optical Physics and Material, Vol. 11, 2002, pp. 31–48) has been known in the art. According to this technique, terahertz waves are generated by pumping ZnTe with ultrashort light pulses of 0.8 μm and are detected by sampling terahertz waves incident on ZnTe with ultrashort light pulses of 0.8 μm. Terahertz waves can be generated and detected with ZnTe and 0.8 μm ultrashort light pulses because the interactive length that allows phase matching in ZnTe Is relatively large between 0.8 μm ultrashort light pulses and terahertz wave pulses. The interactive length that allows phase matching is hereinafter also referred to as “coherent length”.
However, 0.8 μm ultrashort light pulses are generated by a large-scale Ti:sapphire laser system in a free space and are thus rarely applicable to optical communication fibers. Moreover, the device that generates and detects terahertz waves is also large and requires complicated optical alignment. Furthermore, in order to generate and detect terahertz waves using ZnTe and ultrashort light pulses of 1 to 2 μm in the optical communication band, the thickness of ZnTe must be reduced to 100 μm or less to achieve phase matching. Thus, it has been difficult to ensure effective coherent lengths to achieve high generation and detection efficiencies. In other words, crystals having EO effects that can highly efficiently generate and detect terahertz waves using ultrashort light pulses in the optical communication band have not been known in the art.